Vertebrate Flight



The first vertebrates to evolve true flight were the pterosaurs, flying archosaurian reptiles. After the discovery of pterosaur fossils in the 18th century, it was thought that pterosaurs were a failed experiment in flight, or that they were simply gliders, too weak to fly. More recent studies, including work done by UC Berkeley's Dr. Kevin Padian, have revealed that pterosaurs were definitely proficient flyers, and were no evolutionary failure; as a group they lasted about 140 million years (about as long as birds have)!

Pterosaurs are thought to be derived from a bipedal, cursorial (running) archosaur similar to Scleromochlus in the late Triassic period (about 225 million years ago). Other phylogenetic hypotheses have been proposed, but not in the context of flight origins. The early history of pterosaurs is not yet fully understood because of their poor fossil record in the Triassic period. We can infer that the origin of flight in pterosaurs fits the "ground up" evolutionary scenario, supported by the fact that pterosaurs had no evident arboreal adaptations. Some researchers have proposed that the first pterosaurs were bipedal or quadrupedal arboreal gliders, but these hypotheses do not incorporate a robust phylogenetic and functional basis. The issue is not yet closed.

Pterosaur wing

A generalized pterosaur wing (hum= humerus, r= radius, u= ulna, mc= metacarpus, pt= pteroid, c= carpus, I-IV= numbered digits). Adapted from Padian 1985.

Pterosaur sternum
A generalized pterosaur pectoral girdle (sc= scapula, cor= coracoid, hum= humerus, ster= sternum). Adapted from Padian 1985.
The pterosaur wing (shown above) was supported by an elongated fourth digit (imagine having a "pinky finger" several feet long, and using that to fly!). Pterosaurs had other morphological adaptations for flight, such as a keeled sternum (at left) for the attachment of flight muscles, a short and stout humerus (the first arm bone), and hollow but strong limb and skull bones. Pterosaurs also had modified epidermal structures that were wing-supporting fibers, and others that possibly formed hairlike structures to provide insulation. Bird feathers are analogous to the wing fibers of pterosaurs, and both are thought to possibly have been evolved originally for the purpose of thermoregulation (which implies, but does not prove, that both pterosaurs and the earliest birds were endothermic). Pterosaurs also had a bone unique to their clade. It is called the pteroid bone, and it pointed from the pterosaur's wrist towards the shoulder, supporting part of the wing membrane. Such a novel structure is rare among vertebrates, and noteworthy; new bones are unusual structures to evolve — evolution usually co-opts bones from old functions and structures to new functions and structures rather than "reinventing the wheel."

Early pterosaurs (such as Dimorphodon) had long tails that assisted balance, but later pterosaurs (such as Pterodactylus) had no tails, and so may have been more maneuverable flyers. The most derived pterosaurs, such as Pteranodon and Quetzalcoatlus, were so large that soaring was the only feasible option; these were the largest flyers ever known to cast a shadow on the Earth's surface.

More About Pterosaurs

Three Solutions to Flight
   Avian Flight
   Chiropteran Flight

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